But these are pretty minor quibbles. And the way this is set up, a CCS will definitely cause the EQ to be off-spec. If you retrim, the sensitivity of EQ to tube parameters will be even worse. And CCS will reduce the bandwidth.

Everyones' demands and expectations are different of course but...if you care about vinyl a correct RIAA correction (no pun intended) is first on your list.
A correction that is going to deviate from that norm whenever the tubes age or when you use a different brand is just not cutting it in my book.
And, having to trim filter caps and Rs in that department requires more knowledge than the average DIY posesses let alone let alone the assumption of them having a decent inverse RIAA network.

CCS will reduce the bandwidth noticeably on a 12AX7A input tube
for the simple reason that it optimises gain.
Nothing ever comes free and tacking on a CCS on a phono preamp that was not designed for it is definitely going to mess up the deviation from RIAA correction.

If the guys at bottlehead haven't realised this yet then I question their ability as designers.

Quote:

1. 12AX7s. Not a great tube in the first hole for reasons of noise and bandwidth.

While I agree that there are better tubes out there, a manufacturer, even only just for a kit or two, is rather restricted in his choices.
If and when the operating points + the RIAA correction are chosen carefuly neither noise floor nor bandwidth are limited at all.

Let's not forget that this RIAA filter is there to reduce highs and boost bass so if we obtain a 30KHz+ open loop here, we're O.K. when using some active EQ.
For passive EQ filters a tube such as the wideband 6DJ8 may be a better choice.

I'm not saying it's easier to use a 12AX7A but hardly any more difficult than taming the equally silly 6DJ8 in this application.
Meaning that both need a thorough understanding of their idiosyncracies to sound fine.
:cool

Nothing ever comes free and tacking on a CCS on a phono preamp that was not designed for it is definitely going to mess up the deviation from RIAA correction.

If the guys at bottlehead haven't realised this yet then I question their ability as designers.

pedroskova said:

Quote:

Mr. Joppa designed the RIAA network to accept the ccs with no changes(the riaa curve is barely effected).

I'm confused. Are the two statements talking about different things? If not, then it looks like the RIAA circuit was designed keeping a future CCS in mind (assuming it is possible to design that way), right? Just trying to understand if the two of you are talking about the same thing or not.

For a get-started circuit, I'd drop the idea of trying to do anything fancy. There's plenty of time for that later on. I'm very happy with my passive-eq tube-FET hybrid circuit with DC servo'ed output, but I wouldn't recomend it to a novice builder, especially because it uses such hard-to-get tubes.

Start simple. Build one of the many PAS-3 variants out there. You're limited to 12AX7s, but feedback EQ is going to be much less dependent on tube parameters. And they sound just fine if you take some care in parts choice (that doesn't mean fashion parts!) and construction.

One step up in fanciness is Joe Curcio's circuits, with which people have had excellent success.

Interesting that you quoted my statement up to a point but left out the next question I asked Here's what I said:

Quote:

If not, then it looks like the RIAA circuit was designed keeping a future CCS in mind (assuming it is possible to design that way)

So what you're saying is that it's not possible to build such a circuit. Or, it is, but then the compromise one has to settle for is reduced RIAA accuracy. Thanks, that is the information I was looking for.

When you change to a CCS, you change the source impedance and bandwidth of the stage. If the load is the passive EQ, the EQ will change. The only way to avoid this with a passive EQ is to buffer the driving stage, retrim the EQ to account for the changed source Z and bandwidth, or chuck the passive circuit and go for feedback EQ. As you might gather, I'm pretty lukewarm on retrofitting a CCS. It will not benefit a properly designed circuit.

I would agree in principle with Frank- this is a low level stage with lousy PSRR- noise and stability of the power supply rails are critical. Low noise and accurate EQ are the sines qua non of a good phono stage.